The field of the invention is electrical contacts and electrical switches that use such contacts.
Electrical switches have now been in general use for over a hundred years, and have become reliable and commonplace for household and other uses. In applications where higher power, current or voltage is in use, however, some problems of switches have also become commonplace. These problems can include wear and erosion of switch contacts, usually when the switches and contacts are in use for many thousands of operations over an extended period of time.
Erosive wear on contacts may occur when a dc switch is being either opened or closed, but is generally most severe when the switch is being opened and the switch contacts draw apart. When the contacts draw apart, the reduction in contact pressure and resulting increase in resistance between the contact surfaces leads to a significant increase in temperature in the zone of contact. Localized melting of the contact material may occur, and an arc may form between the contacts. An arc may cause erosion of the contacts, and eventually lead to pitting, excessive wear, and even mechanical failure. Another possibility for erosive wear occurs when the switch does not reliably hold the contacts in position and “chatter” or high frequency movement between the contacts causes momentary opening and additional arcing.
As a result of these problems, switches may be designed to lessen the arcing and chatter that causes erosive wear and failure. Thus, switch contacts may be made from sturdy and reliable designs with high copper content for minimum resistance and maximum reduction of localized heat. Contacts may be alloyed with silver or other metals to minimize the effects of arcing. Contacts may also be spring-loaded, i.e., kept in contact by a spring loaded in compression as part of the contacts circuit.
An example of efforts to minimize wear and pitting is disclosed in U.S. Pat. No. 5,221,816. This patent discloses a plunger switch using a bent wiper contact that is held in compression between a common terminal and a normally closed (NC) or a normally open (NO) terminal. The wiper is in the general shape of a U or V that has been bent even further wide open. The outer legs are mounted on or insert molded into a plastic retainer that moves with the plunger. However, in use, these contacts are easily eroded by arcing and fail in service. Without being bound to any particular theory, it is believed that the wipers may be unevenly loaded between the contacts. It is also possible that their relatively thin, two-dimensional nature does not provide enough contact material compared to what is needed to survive erosive wear.
In another example, U.S. Pat. No. 7,060,917 discloses a plunger switch that uses a hollow bushing or plate to make or break electrical contact between upper NO contacts and lower NC contacts. Internal springs urge the plunger upward for making contact with the NO contacts while the exterior of the switch is molded to include a hook-engaging portion so that a hook may be used to keep the switch actuated (closed). Engagement of the internal portions of the contacts depends on proper assembly and the alignment and force of the springs. Over time, it appears that this switch also will be subject to uneven wear and arcing as the springs relax and as the plate is deformed.
What is needed is a switch with contacts that are resistant to arcing and with sufficient heft and mass to resist nominal pitting or wear resulting from operation of the switch. The contacts should have low resistance to current and voltage and high reliability.